A water fog (as opposed to ice fog), whose temperature is lower than 0.degree. C., is called a "supercooled fog". The supercooled fog layer that develops in winter often accumulates pollutants and deflects the sunshine. This causes cooling which further induces problems like more home heating fuel consumption and consequently more pollution generation, air pollution accumulation, and poor visibility over the airport and highways, in addition to worsening conditions for people with respiratory problems and those with mental instabilities and illnesses. Particulate air pollutants are effective center nuclei of fog droplet formation. Due to increased size of fog droplets compared with the dry nuclei, they are more easily captured in the human lungs, making the pollutants more lethal. This problem is not well appreciated by researchers who are measuring the air pollution level.
The only known technique of weather modification that is truly proven is the clearing of supercooled fog, although there still remain a number of technical problems. Dry ice seeding has been routinely carried out at a number of airports throughout the world. Despite the fact that the method is not very scientific and efficient, with strenuous effort, it has been marginally successful in keeping airports open during fog episodes. Under awkward meteorological conditions like fast drifting dense fog, failure in clearing is common.
At temperatures below 0.degree. C., water gives higher vapor pressure than ice. If an ice crystal appears in a supercooled fog, due to the pressure difference between them, water vapor evaporates from supercooled fog droplets and deposits onto the ice crystal surface with associated heat generation. As a result, first the ice crystal becomes larger and the fog droplets within the vicinity evaporate and disappear, and secondly the ice crystal falls out.
Cloud seeding requires introduction of a large number of ice crystals into supercooled clouds and fogs. When a supercooled fog is seeded, the ice crystals thus grow by evaporating the fog, i.e., converting a large number of small fog droplets into a fewer number of large crystals. When the crystals become large enough, they will fall out. The effect of cloud seeding to fog visibility and therefore sunshine transmission is twofold:
(1) Change from a large number of small fog droplets to a much smaller number of large crystals. For a given mass of condensate, the visibility is approximately proportional to the size. PA1 (2) Ice crystal fallout. PA1 (a) a source of liquid carbon dioxide, PA1 (b) nozzle means oriented to introduce liquid carbon dioxide into the fog, PA1 (c) a conduit means for conveying the liquid carbon dioxide from the source to the nozzle means, PA1 (d) a vehicle means for conveying the nozzle means along the ground level so as to distribute liquid carbon dioxide into the fog while moving along the ground.
When the ice crystals grow by evaporating fog droplets, the process proceeds under approximate conservation of the mass of the condensed phases, i.e., water and ice. The visibility (or sunlight penetration) is roughly proportional to the product of the number concentration of particles and the cross-sectional area which is proportional to r.sup.2 where r is the particle radius. From the mass conservation, n.varies.1/r.sup.3. Then, EQU VISIBILITY.varies.1/(nr.sup.2).varies.r.varies.(1/n).sup.1/3. (1)
The fall velocity, w, of ice crystals in question here obeys the so-called Stokes law, or EQU w.varies.r.sup.2 .varies.1/(n).sup.2/3. (2)
From these two relationships, it is clear that a larger size, with fewer number of crystals, is advantageous for visibility increase, instead of a smaller size with larger numbers.